Atomic force microscopy measurements of contact resistance and current-dependent stiction

ABSTRACT

A modified atomic force microscope (AFM) is used to perform contact resistance and/or current-dependent stiction measurements for conductive thin films at controlled values of applied force. The measurements are preferably performed under conditions approximating the operation of the thin films as electrodes in microswitch array fingerprint sensors. A first, planar thin film is contacted with a second, curved thin film deposited over a round ball having a diameter of a few microns to a few tens of microns. The second film is preferably a coating deposited over the ball and over the arm controlling the ball motion. The coating deposited over the arm provides an electrically conductive path to the contact surface of the ball.

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No.09/571,765 filed May 18, 2000 entitled “Method and Apparatus forPressure Sensing,” and U.S. patent application Ser. No. 10/038,505 filedDec. 20, 2001 entitled “Fingerprint Sensors using Membrane SwitchArrays.”

FIELD OF THE INVENTION

The invention relates to systems and methods for measuring contactresistance and/or stiction, and in particular to methods of evaluatingcontact materials for microswitches such as fingerprint sensor switches.

BACKGROUND OF THE INVENTION

The fingerprint sensing industry uses several different conventionaltechnologies to capture images of an individual's fingerprints. Twoprominent technologies are optical-based sensors and capacitance-basedsensors. In a typical optical sensor, a light source, lenses and a prismare used to image the ridges and valleys on a fingerprint, based ondifferences in the reflected light from the features. Conventionalcapacitance sensors include two-dimensional array of capacitors definedon a silicon chip, and fabricated by semiconductor CMOS processing. Theindividual sensors on the chip form one plate of the parallel platecapacitor, while the finger itself, when placed on the array, acts asthe second plate for the various localized sensors. Upon contact withthe array of sensors, the individual distance from each sensor to thecorresponding point on the skin above the sensor is measured usingcapacitive techniques. The difference in distance to skin at the ridgesand valleys of a fingerprint identifies the fingerprint.

Capacitive and optical sensors can be sensitive to oils or grease on thefinger and to the presence or absence of moisture on the finger. Inaddition, the ambient temperature can affect these sensors at the timeof sensing. Under hot or cold conditions, capacitive sensors can provideerroneous readings. Finally, most sensors have abrasion resistantcoatings. The thickness of the protective coating can affect themeasurements. The combined effect of these variables can result indistorted fingerprint images. Finally, in the case of silicon chip basedfingerprint sensors, the placement of the finger directly onto thesilicon increases the risk of electrostatic discharge and damage to thesensor.

The above-referenced U.S. patent application Ser. Nos. 09/571,765 and10/038,505 describe systems and methods for performing texture (e.g.fingerprint) measurements using switch arrays. A fingerprint sensor forperforming such measurements can include tens of thousands of miniatureswitches arranged in an x-y array. Each switch includes a lowerelectrode, and an upper electrode disposed over the lower electrode.Depending on the force applied on the switch, the upper electrode can beseparated from the lower electrode, or can establish electrical contactwith the lower electrode. The state (open or closed) of each switchindicates whether a fingerprint ridge or valley is positioned above thatswitch. A map showing the distribution of switch states over the sensorarea can thus be used to identify a fingerprint positioned over thesensor.

In a representative implementation of such a fingerprint sensor, eachswitch extends over a few tens of μm along the x- and y-directions, andis capable of changing state upon the application of a loadcorresponding to a few mg of mass. Such a switch can include upper andlower electrodes formed by thin films having thicknesses on the order oftenths of micron to a few microns. The performance of such a switch candepend to a significant extent on the properties of the thin filmsdefining the electrodes. Systems and methods for systematicallyevaluating such thin films could be of great benefit to the design ofmore accurate and reliable switches for texture sensing and otherapplications.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for performingmeasurements that are useful in determining materials that are useablefor various applications and particularly contact materials useable forswitches of different types, and more particularly for microswitches,using an atomic force microscopy measurement. A measurement ormeasurements, for instance of contact resistance between a pair ofopposing thin films, is performed at a first controlled applied forcevalue between the thin films. Each of the thin films is formed by amaterial being considered for use in the application, switch ormicroswitch of interest. A subsequent atomic force microscopymeasurement is used to obtain a current-dependent stiction force valuebetween the thin films. The contact resistance, the stiction force, orother measurement values, either separately or in combination, areemployed to determine materials that are suitable for the application,and particularly contact materials that will contact each other inswitches or microswitches. Of particular interest is measuringcharacteristics of thin film materials.

In one aspect of the invention to perform the measurements, a first filmof a first contact material is disposed on a planar substrate. A secondfilm of a second contact material is disposed on a rounded piece. Theradius of curvature of the rounded piece along a contact surface betweenthe first film and the second film is preferably higher than 10 μm andlower than 100 μm. Measurements between the first film and the secondfilm are then performed at a controlled applied force value between thefirst thin film and the second thin film, preferably not exceeding 1milliNewton (mN).

Other aspects and advantages of the present invention will becomeapparent hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and advantages of the present invention willbecome better understood upon reading the following detailed descriptionand upon reference to the drawings where:

FIG. 1 is a schematic diagram of a modified atomic force microscopyapparatus for performing contact resistance and/or stictionmeasurements, according to the preferred embodiment of the presentinvention.

FIG. 1-B is a schematic diagram of a contact resistance measurementcircuit according to an embodiment of the present invention.

FIG. 2 shows measured and computed contact resistance values for severalthin films, according to an embodiment of the present invention.

FIGS. 3-A and 3-B show computed contact resistance values for Ru-Mocontact surfaces, according to an embodiment of the present invention.

FIG. 4 shows stiction force values for two Au—Au contact surfaces,according to an embodiment of the present invention.

FIGS. 5-A and 5-B are side view illustrations of the deposition of aconductive film on a movable member, before and after the deposition,respectively, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, it is understood that each recited elementor structure can be formed by or be part of a monolithic structure, orbe formed from multiple distinct structures. An electrically-dependentor current-dependent stiction force value is a value of a stiction forcebetween two surfaces measured after or during a passage of electricalcurrent through the contact interface between the two surfaces. Unlessexplicitly specified otherwise, a thin film is understood to be a filmhaving a thickness of less than about 1 micron. The term “material” isunderstood to encompass composite and/or alloy structures comprisingmultiple chemical elements or compositions. The terms “upper” and“lower” are used to describe relative positions, and do not necessarilyrefer to the direction of gravity. A set of elements is understood toinclude one or more elements. A plurality of elements is understood toinclude two or more elements. Any recitation of an element is understoodto refer to at least one element. Unless specified otherwise, anyrecitation of a first element and a second element (e.g. first andsecond applied forces) is understood to allow for a first element equalto the second element. For example, a first material of a pair ofcontact materials need not be necessarily different from a secondmaterial of the pair of contact materials.

The following description illustrates aspects of the invention by way ofexample and not necessarily by way of limitation.

The design of microswitches used for fingerprint recognition and otherapplications would benefit from systematic methods of evaluating thethin metal films that define the contact surfaces, at contact forcevalues and electrical current conditions corresponding to the operatingconditions of the switches. Parameters of particular interest for theoperation of such switches are contact resistance and stiction.Selecting electrode materials with low contact resistance and stictionforces facilitates discriminating between the open and closed states ofthe switches, and reduces the likelihood that switches remainpermanently closed. Microswitch applications where such contact materialevaluation techniques would be of benefit include communications, powerelectronics, indicators and instrumentation, industrial automation,automotive applications, and applications involving extreme operatingconditions, such as space and defense applications.

The importance of providing low contact resistance and low contactstiction increases as the microswitches are miniaturized to ever-smallersizes, and operate at smaller forces. For micro-switches, the force withwhich surfaces mate can be in the microNewton to milliNewton range. Forclean surfaces in Asperity contact, the asperities along the surfacesconstrain the flow of current, thus creating contact resistance.Contamination along the contact surfaces can introduce an additionalresistance at the contact interface. Current-dependent stiction is theforce with which two mating surfaces are held, due to micowelding, afterthe application of force and current. Stiction can occur even in theabsence of current flow, due to mechanical alloying resulting fromapplied force. The stiction effect is significantly more pronounced withthe passage of current, as the passing current induces heating ofasperities along the contact interface and promotes micro-welding. Ifthe stiction force exceeds the restoring force of the micro-switch, themicro-switch can become permanently closed or stuck, and can thus stopfunctioning. As switches are miniaturized, their restoring forces oftendecrease, and their vulnerability to stiction-induced failure increases.

Both contact resistance and stiction forces are strongly dependent onthe geometry of contact. To simulate real contacts, the evaluationmethod ideally reproduce the real microswitch, geometry. For fingerprintsensors, for example, where contact is between a flat surface (typicallythe flat lower electrode) and a slightly rounded surface (typically theflexing upper electrode), and the contact area is sub-micron in spatialdimension, sharp probes or macroscopically large probes would not yieldaccurate results reflecting the operation of the microswitch. Thepreferred embodiment of this invention aims to better reproduce thegeometry of real microswitches, as well as provide accurate andconvenient ways of measuring contact resistance and stiction.

Consider a typical load applied by an individual's finger on amicroswitch array sensor, as described in further detail in theabove-referenced U.S. patent application Ser. No. 10/038,505. Consideran applied load in the range of 100-500 grams, a fingerprintapproximately 15 mm×15 mm in general diameter, and an array of switcheswith total dimensions of 15 mm×15 mm. The spacing between typicalfingerprint ridges is on the order of 400 μm. If the switches are placed50 μm apart on a two dimensional x-y grid, an array of on the order of300×300 switches would be suitable for covering a sensor, surface areaof 15 mm×15 mm. There are a total of 90,000 sensors in such an array,and the applied load from the fingertip can be assumed for simplicity tobe distributed over these 90,000 sensors. As a first orderapproximation, one can assume that the area of the ridges is equal tothat of the valleys. Thus, approximately 45,000 sensors bear the appliedload. If one conservatively assumes an applied load of 90 grams from thefingerprint, then each cell bears an approximate load of about 2 mg.Such a load corresponds approximately to an applied force on the orderof tens of microNewtons (μN), e.g. 10-30 μN.

FIG. 1 shows a schematic side view of a contact resistance and stictionmeasurement apparatus 20 according to the preferred embodiment of thepresent invention. Apparatus 20 is preferably a modified atomic forcemicroscope (AFM). Apparatus 20 can be made by performing modificationsas described below on a commercially available AFM, such as aVeeco/Digital Instruments Dimension-series AFM. Apparatus 20 is capableof performing contact resistance measurements, stiction measurements,and/or other measurements over length scales in the micron and submicronranges. In particular, the contact area defined by apparatus 20 can havean in-plane extent of microns to fractions of a micron. Apparatus 20 isalso preferably capable of generating and measuring controlled appliedforces and changes in applied forces in the range of microNewtons totens of microNewtons.

Apparatus 20 comprises a fixed support 22, a thin film conductive sample24 mounted on support 22, and a rounded surface, conductive movableupper member 26 capable of contacting sample 24. Conductive sample 24 ispreferably a thin metal/alloy film having a thickness of hundreds tothousands of Ångstroms. Exemplary materials for sample 24 include,without limitation, gold, chromium, molybdenum, ruthenium, iridium,indium tin oxide (ITO), platinum, palladium, or any other conductivematerial to be evaluated. The thickness and surface properties/treatmentof sample 24 preferably correspond to the actual thickness and surfaceproperties of at least one of the contact films to be used in themicroswitches of interest, such that the data measured for sample 24 isindicative of the properties of the film(s) to be used in themicroswitches of interest. Sample 24 can be a multilayer film, e.g. abilayer film. Such a multilayer layer can include a conductive outerlayer, and an inner layer or layers formed by conductive or insulativematerials such as metals, polymers, or other desired materials.

Apparatus 20 further comprises control/acquisition components 30electrically and mechanically connected to sample 24 and member 26.Control/acquisition components 30 include mechanical and electroniccontrol components for controlling the relative positions of sample 24and member 26. Control/acquisition components 30 further includeacquisition electronics for acquiring data indicative of contactresistance and stiction values between sample 24 and member 26. Theacquisition electronics preferably include a voltage or currentmeasurement unit for determining the current passing between sample 24and member 26 and/or other data indicative of the contact resistancebetween sample 24 and member 26. The acquisition electronics preferablyalso include a force setpoint indicator for measuring the applied forceapplied between sample 24 and member 26. In particular, the forcesetpoint indicator is used to record the force applied to member 26 at atimepoint during a retraction of member 26 when the current betweensample 24 and member 26 drops to zero, which will occur when the sample24 and the member 26 are no longer in contact, as described in furtherdetail below.

Support 22 preferably comprises a conventional fixed AFM chuck 32, and asample substrate 34 mounted on chuck 32. Substrate 34 is preferably asilicon or glass substrate having a thickness of about 100 to 1000microns, on which conductive sample 24 is deposited. Substrate 34 mayinclude a conductive layer deposited onto a glass or silicon support, incontact with sample 24. Such a substrate conductive layer may be used toreduce the parasitic resistance of the measurement circuit, asillustrated below. The upper surface of sample 24, opposite substrate34, forms a contact surface for contacting upper member 26.

Upper member 26 comprises an elongated rigid cantilever arm 38, and arounded end piece 40 mounted at the distal end of arm 38. A conductivecoating (sample) 44 extends over end piece 40 and arm 38, for providingan electrical conduction path from the proximal end of arm 38 to acontact surface defined along end piece 40. Conductive coating 44 ispreferably formed by a material of interest to be evaluated in contactresistance and/or stiction measurements to be performed with apparatus20. Such a material may include, without limitation, gold, chromium,molybdenum, ruthenium, iridium, indium tin oxide (ITO), platinum,palladium, or any other conductive material to be evaluated. Coating 44can be formed by a multilayer-film, as discussed above with reference tosample 24. Coating 44 can be a bi-layer including, for example, an innerlayer providing a low resistance to current flow, and an outer layerformed of the material of interest. The inner layer can serve to reducethe parasitic resistance of the measurement circuit. Coating 44 can beidentical in composition to sample 24. Coating 44 preferably has athickness of hundreds to thousands of Ångstroms.

The proximal end of arm 38 is connected to control/acquisitioncomponents 30. Control/acquisition components 30 control a linear,vertical motion of arm 38. Controlling the position and motion of arm 38allows controlling the contact force applied by end piece 40 to thecontact surface of conductive sample 24. The inner part of arm 38 ispreferably made of stainless steel or another rigid composition. Thegeneral shape (major axis) of arm 38 can make an acute angle, e.g. about10-15°, with the horizontal direction.

End piece 40 is preferably a smooth, spherical ball having a diameter of10-100 μm, in particular about 40-60 μm, for example about 50 μm. Ingeneral, balls having diameters or radii of curvature of between as lowas 1 μm and as high as 500 μm may also be used. The geometry of a spherecontacting a flat surface is well suited to using the Hertz equations ofcontact mechanics. The Hertz equations, in combination with contactresistance theory, can be used to quantitatively analyze the contactresistance properties of the materials and switches of interest.Furthermore, a spherical (Hertzian) shape for end piece 40 can allowsimplifying the theoretical characterization of the contact surface. Endpiece 40 preferably comprises an inner core 46, and the part ofconductive coating 44 disposed over inner core 46. Inner core 46 ispreferably made of a hard material such as tungsten, titanium, orsilica. The part of conductive coating 44 disposed over the lower partof inner core 46 defines a rounded, preferably spherically-curved,contact surface 48 capable of contacting sample 24. As described above,conductive coating 44 preferably extends over a large enough area ofupper member 26 so as to provide an electrically-conductive path betweencontact surface 44 and the acquisition electronics ofcontrol/acquisition components 30.

As is apparent, it is the shape of end piece 40 along contact surface48, rather than the overall size of end piece 40, that primarilydetermines the contact resistance characteristics of end piece 40.Consequently, end pieces that are not generally spherical or rounded,but have rounded (e.g. spherical, cylindrical, ellipsoidal, etc.)contact surfaces may be used. The relevant parameter for such an endpiece is the radius of curvature along the contact surface, rather thanthe overall size of the end piece. For a 60 μm spherical ball, theradius of curvature along the contact surface is 30 μm. For a rounded,non-spherical end piece, the radius of curvature along the contactsurface may vary within a range.

End piece 40 is preferably attached to arm 38 by a non-conductiveconventional epoxy. Alternatively, end piece 40 may be attached to arm38 using a conductive epoxy such as silver epoxy, other types ofadhesives, or mechanical fasteners. It was empirically observed thatcommercially available conductive epoxies such as silver epoxy havepoorer adhesive properties than available non-conductive epoxies. Suchconductive epoxies may also introduce relatively high resistances in theconductive path between the contact surface and the acquisitionelectronics. Moreover, core 46 and arm 38 may include native surfaceoxides, whose resistances may be significant even if the resistance of aconductive epoxy is relatively low. Thus, applying an externalconductive coating to member 26 may be desirable even if a conductiveepoxy were used to attach end piece 40 to arm 38.

To perform measurements of the contact resistance between lower sample24 and upper sample 44, upper member 26 is lowered until end piece 40contacts sample 24. The position of end piece 40 is then controlled toapply desired forces to sample 24. Preferably, the applied forces arebetween microNewtons to milliNewtons, and in particular on the order oftens of microNewtons. The applied force is preferably controlled with anaccuracy on the order of microNewtons to tens of microNewtons.Generally, the applied forces hare selected according to the forceexpected to be applied to the microswitch of interest during theoperation of the microswitch. For example, the applied forces can beselected to be in a range centered around an average applied forceexpected during the operation of the microswitch. The magnitude of theapplied force is measured using conventional AFM components ofcontrol/acquisition components 30. The acquisition electronics ofcontrol/acquisition components 30 are used to record electricalparameter (e.g. voltage or current) values indicative of the contactresistance between end piece 40 and sample 24 at various values ofapplied force between end piece 40 and sample 24. Contact resistancevalues are then computed from the recorded electrical parameter values.

FIG. 1-B shows a schematic illustration of an exemplary contactresistance detection circuit 50 according to an embodiment of thepresent invention. Circuit 50 includes several resistances arranged inseries outside of the acquisition electronics of apparatus 20, asdescribed below. A contact resistance 52 (R_(c)) is the resistanceintroduced by the interface between samples 24, 44. A first parasiticresistance 54 (R_(p1)) is the resistance of upper member 26 and theother part of the electrical path between contact surface 48 and theacquisition electronics. A second parasitic resistance 56 (R_(p2)) isthe resistance of sample 24 and the other part of the electrical pathbetween sample 24 and the acquisition electronics. The parasiticresistances 54 and 56 can be thought of as an equivalent, singleparasitic resistance R_(p) incorporating all the parasitic resistancesof circuit 50. In typical present applications, the parasitic resistanceof circuit 50 can vary from less than one ohm to several ohms. Theparasitic resistance can depend on film resistivity, thickness,structure (e.g. bilayer or multilayer), and other parameters.

The parasitic resistances 54, 56 can be determined empirically in acalibration measurement. Such a calibration measurement can be takenusing the system described above at a high applied force, or by usingexternal probes. The force used in the calibration measurement ispreferably at least about an order of magnitude higher than the forceused for the contact resistance measurements. For example, for contactresistance measurement forces in the range of 1-50 microNewtons, acalibration applied force of at least about 500 microNewtons ispreferably used. A cantilever arm having a high enough spring constantto permit the application of such forces is then used. A calibrationmeasurement can also be performed by using thick gold films along thecontact surface, or by using bi-layer films. A parasitic resistancecalibration measurement may not be needed for performing measurements ofchanges in contact resistance as a function of applied force, since theparasitic resistance of the circuit should not change with appliedforce. The calibration measurement is used to obtain an precise value ofthe resistances in the measurement circuit, including leads, equipmentinternal resistances, external resistances etc., since the presentinvention measures such small and accurate values of contact resistance.This high force is in the calibration measurement to essentially ensureperfect contact and, therefore, near zero contact resistance, at whichpoint isolation of the rest of the measurement circuit resistances canoccur.

Circuit 50 further includes several components which are part of theacquisition electronics of apparatus 20. A constant voltage source 60has one terminal connected to sample 24, and the other terminalconnected to ground. Voltage source 60 preferably generates a DCvoltage, preferably between 0 and 5 V. In a present implementation,voltage source 60 is a 3.5 V d.c. power supply. The internal resistanceR_(i) of voltage source 60 is shown at 62. In a present implementation,internal resistance 62 is 1100Ω. A voltage meter 68 is connected acrossa high, known measurement resistance 64. Measurement resistance 64 isconnected between upper member 26 and the ground terminal of voltagesource 60. Measurement resistance 64 is preferably equal to theequivalent resistance of the rest of circuit 50.

For the measurement circuit shown in FIG. 2, the contact resistanceR_(c) between sample 24 and sample 44 is $\begin{matrix}{{R_{c} = {{R_{m}( {\frac{V}{U} - \frac{R_{m} + R_{i}}{R_{m}}} )} - R_{p}}},} & \lbrack 1\rbrack\end{matrix}$

where U is the voltage measured by voltage meter 68, and R_(p) is thetotal parasitic resistance R_(p)=R_(p1)+R_(p2). For the resistancevalues in the implementation described above, the resolution achievedwith circuit 50 is on the order of a few ohms. The sensitivity of themeasured voltage U across R_(m) is highest when R_(m) is matched (orequal) to R_(i), assuming R_(i)>>(R_(c)+R_(p)). Improved resolutions canbe achieved by using different measurement circuits, such as for examplea current-measuring circuit including a logarithmic current amplifier.

FIG. 2 illustrates exemplary contact resistance data taken using theapparatus described for one Au—Au and two Au—Cr contact surfaces, andcomputed contact resistance data generated using a tunneling model.Curves 220, 222 and 224 show the measured Au—Au, Au—Cr, and Au-etched Crdata, respectively. The star-shaped data points illustrated at 226 werecomputed using electron tunneling theory assuming a work function thatvaries with applied force, to enable a match to the data. The squareshaped data points shown at 228 were computed for a fixed work functionand fixed film thickness. A gold-coated tip and gold-coated siliconsubstrate were used for the gold-on-gold measurements. A gold-coated tipand chromium-coated silicon substrate were used for the gold-on-chromiummeasurements. The gold films were deposited as described in detailbelow. A physical vapor deposition (PVD) process was used to deposit thetwo chromium films.

No oxide formed on the gold films. Consequently, the contact resistanceat the Au—Au film interface was relatively low and independent ofapplied force. The Cr film corresponding to the curve labeled “Au-etchedCr” was etched after deposition by dipping into an acetic acid/hydrogenperoxide mix. The etching process resulted in the formation of arelatively thick (about 2 nm) oxide layer on the Cr film surface.Consequently, the contact resistance as a function of applied forcestayed relatively high, and decreased only for applied force valuesabove 20 μN. The Cr film corresponding to the curve labeled “Au—Cr” wasexposed to air but not etched, and had a thinner oxide film. The contactresistance for the Au—Cr interface decreased rapidly for applied forcesunder 10-20 μN.

FIG. 3-A shows computed contact resistance data 320 as a function ofapplied load for a Ru—Mo interface, with RuOx and MoOx contamination(oxide films) on the metal surfaces. FIG. 3-B shows computed contactresistance data as a function of applied load for a Ru—Mo interfacehaving no contamination and RuOx contamination only, respectively. Curve322 shows data for no surface contamination, while curve 324 shows datafor RuOx contamination only. The data was computed for a contact surfacedefined at the interface between a sphere and a planar film, using theHertz equations and the theory of contact resistance. For furtherinformation on contact resistance theory see for example R. Holm,“Electrical Contacts: Theory and Application,” New York,Springer-Verlag, 1967.

Referring back to FIG. 1-A, apparatus 20 can also be used to performmeasurements of electrically-dependent (current-dependent) contactstiction forces on samples of interest. When two surfaces come intointimate contact, the two surfaces can become attached to each other.The stiction phenomenon can be thought of as microwelding or coldwelding caused by formation of bonds between atoms along the twosurfaces. The stiction force between the two samples is the forcerequired to separate the samples after the samples are brought intocontact using an applied force. The microwelding phenomenon and theassociated stiction force can depend on the passage of current throughthe contact surface. The passage of electrical current can causelocalized heating of the samples along the contact surface, thusenhancing the microwelding process and increasing the stiction force.For example, such localized heating can lead to melting of samplemicroasperities along the contact surface.

To perform a measurement of a current-dependent stiction force, uppermember 26 is lowered until ball 40 contacts sample 24 and applies adesired level of downward force to sample 24. A desired stictionmodulation voltage is applied using voltage source 60, such that acontrolled amount of current passes through the contact interfacebetween samples 24, 44. The stiction modulation voltage is preferablychosen such that the current passing through the contact interfacebetween samples 24, 44 is equal to or similar in magnitude (e.g. withina factor of ten) of the current expected during an operation of amicroswitch of interest. The current is preferably between 5 μA and 5mA, for example between 5 μA and 100 μA. Higher currents, on the orderof mA, may be used to simulate short-circuits or other atypicaloperating conditions that may occur for example if the switch issubjected to shocks. In a present implementation using the detectioncircuit 1-B, the stiction modulation voltage applied using voltagesource 60 is about 5 V.

An increasing upward force is then applied to upper member 26. Ifdesired, to prevent arcing between samples 24, 44 when samples 24, 44become separated, the voltage applied by voltage source 60 can bereduced to a stiction-measurement value before the upward force isapplied to upper member 26. The stiction measurement value is lower thanthe stiction modulation value, for example at least an order ofmagnitude lower than the stiction modulation value. In a presentimplementation, the voltage applied using voltage source 60 is reducedfrom a stiction modulation value of about 5 V to a stiction measurementvalue of about 0.2 to 0.3 V.

The force required to separate member 26 from sample 24 (the stictionforce) is then recorded. The stiction force is preferably measured byrecording the force corresponding to a cessation of current flow betweensample 24 and upper member 26, as detected by voltage meter 68.Alternatively, the stiction force can be measured using conventionalmechanical (e.g. piezoelectric) components, in a manner which does notdepend on the passage of current through the contact interface betweensamples 24, 44.

The applied force values for stiction measurements can be (but need notbe) significantly larger than the applied force values for contactresistance measurements. As an illustration, for evaluating amicroswitch such as a fingerprint sensor microswitch, the applied forcesof interest can be forces due to shocks or impacts applied to themicroswitch. If such an applied force leads to a stiction force higherthan the restoring elastic force of the microswitch, the switch canremain permanently closed regardless of whether an external force isapplied subsequently. In designing such a microswitch, the stictionforces resulting from expected shocks to the microswitch are ideallylower than the restoring force of the microswitch. In a presentimplementation, stiction force values are measured for applied forces oftens to hundreds of microNewtons. Depending on the application, suitableapplied forces may range up to tens of microNewtons, 1 mN, or hundreds(e.g. 200) of milliNewtons. In general, the applied force for thestiction force measurements can be selected according to expected impactapplied forces during the operation of the microswitch, for example in arange up to the maximum expected operating impact force. The appliedforces and voltages for the stiction force measurements can also beselected according to the expected normal operating conditions of theswitch of interest, as described above with reference to contactresistance measurements.

FIG. 4 shows measured stiction force data illustrating the dependence ofstiction force on applied force for two Au—Au contacts: a contactbetween two unetched Au surfaces, and a contact between an unetched Ausurface and an etched surface. The data points represented by thesquares 420 are for the etched surfaces, while the data pointsrepresented by the circles 422 are for the unetched, contact surface.The Au films were sputtered from pure targets in a magnetron sputteringsystem. The unetched surfaces were as-deposited. The etched surface wasetched in an acetic acid/hydrogen peroxide mixture. Etching cleaned theAu surface and made it more prone to stiction, as shown. Cleaning the Ausurface of organic and other contaminants by etching can facilitatemicrowelding, and thus increase the stiction force required for a givenapplied force.

Apparatus 20 is preferably made by modifying a conventional,commercially-available atomic force microscope (AFM) as described below.A conventional sharp-tip AFM cantilever is replaced with a cantileverstructure having a flat or rounded contact surface in electricalcommunication with the acquisition electronics of the apparatus. Thecantilever structure can be kept connected to the acquisitionelectronics, rather than grounded. A voltage or current meter can beconnected to the chuck and arm as to as measure a voltage or currentindicative of the contact resistance values of interest. For thepreferred design shown in FIG. 1-A, the hard inner core of spherical endpiece 40 is preferably epoxied to the inner part of arm 38 using anon-conductive epoxy. A suitable cantilever having an epoxied ball atits end can also be obtained commercially, for example fromVeeco/Digital Instruments, Santa Barbara, Calif. The resulting assemblyis mounted into a vacuum system, for depositing conductive coating 44over the assembly. Conductive coating 44 can be deposited by ion beamdeposition, sputtering (physical vapor deposition), chemical vapordeposition (CVD), electroplating, dip-coating, or other such methods.

In a present implementation, coating 44 is deposited by ion beamdeposition. FIGS. 5-A and 5-B illustrate schematically the preferredassembly mounting geometry employed for the deposition of coating 44 byion beam deposition. FIG. 5-A shows member 26 before the deposition ofcoating 44, while FIG. 5-B illustrates member 26 after the deposition ofcoating 44. The arrows 80 denote the direction of motion of the coatingmaterial toward member 26. The interface area along which end piece 40and arm 38 form a most acute angle is shown at 82. As shown in FIGS.5-A-B, end piece 40 can be attached to arm 38 along a major longitudinalsurface of arm 38, rather than along the transverse end surface of arm38.

As illustrated, upper member 26 is mounted within the deposition vacuumsystem such that end piece 40 does not shield the acute-angle interfacearea 82 from the source of coating material. Allowing end piece 40 toshield interface area 82 can lead to a suboptimal electrical connectionbetween the coating area covering end piece 40 and the coating areacovering arm 38. Preferably, upper member 26 is positioned such thatinterface area 82 squarely faces the source of coating material. Such apositioning corresponds to an acute angle between the coating depositiondirection illustrated by arrows 80 and the surface of arm 38 facing thesource of coating material. In a present implementation, the acute angleis preferably between 30° and 60°, in particular about 45°.

The above-described systems and methods allow performing contactresistance and stiction measurements under conditions similar to thosein microswitches suitable for use in fingerprint recognition microswitcharrays, or other microswitches subject to similar forces and operatingconditions. Although AFM devices have found multiple applications,conventional AFM devices are typically used to profile surfaces withhigh resolution. For such applications, typical AFM tips are very sharpand are operated under ultra-low force conditions. Conventional AFM tipscommonly end in a sharp point having a size of tens to hundreds ofnanometers. Many developments in AFM have focused on improving thesharpness of the AFM tip, in order to improve the resolution of thedevice as it scans surfaces of interest. Such ultra-sharp tips areordinarily not suitable for performing contact resistance or stictionmeasurements for typical microswitches. Moreover, typical AFM tips aremade of insulating materials such as diamond, silicon, or siliconnitride. Consequently, a conventional AFM device is preferably modifiedas described above in order to perform contact resistance and/orcurrent-dependent stiction measurements.

AFM devices have also been used to characterize electrical properties ofsamples of interest. In U.S. Pat. No. 6,208,151, Thomas et al. describean AFM having a conductive sharp tip for mapping voltage gradients in aplanar sample. Current is injected from the tip into the sample, andvoltages in the sample plane are measured using a voltmeter. The contactresistance between the tip and the sample can be a nuisance in themeasurements described by Thomas et al., and its effects are compensatedfor by injecting constant current into the sample. In U.S. Pat. No.5,723,981, Hellemans et al. describe employing a sharp tip for measuringvoltages across a biased semiconductor device. Applying a high force forthe measurement minimizes the effect of any contact resistance on thevoltage measurements. In their article “Lateral and Vertical DopantProfiling in Semiconductors by Atomic Force Microscopy using ConductingTips,” J. Vac. Sci. Tech. A, 13 (3):1699-1704, May-June, 1995, De Wolfet al. describe a method of determining the spatial distribution ofcharge carriers in semiconducting structures using an atomic forcemicroscope. A sharp conductive tip is scanned across a sample, and thelocal spreading resistance of the sample is determined from the measureddata. The references cited above focus on determining electricalproperties within a single sample, rather than measuring contactresistances or stiction force values between two sample surfaces.

Bulk (millimeter-size) measurement devices are not normally capable ofcontrolling the applied force finely enough to allow adequate simulationof expected microswitch operating conditions. Bulk measurement devicesmay only be capable of applying relatively high forces, and may not beuseful in controlling small changes in the applied force. In addition,bulk measurement devices having millimeter-size radii of curvature alongthe contact surface can be limited in their spatial resolution.

It is apparent that the above embodiments may be altered in many wayswithout departing from the scope of the invention. For example, an endpiece having a cylindrical, ellipsoidal, or other rounded (e.g.non-spherical) contact surface may be used. A curved, non-spherical endpiece may be used while maintaining the radius of curvature of the endpiece within a desired range along the contact surface. An intermediateconductive structure of surface can be inserted between two electrodesof interest in order to measure a contact resistance of interest. Forexample, an intermediate gold structure may be placed in contact with asurface of interest (e.g. a chromium-oxide surface), and a second goldelectrode may be brought into contact with the gold structure. It isunderstood that other dimensions and materials can be suitable for usewith the present invention. Further, various aspects of a particularembodiment may contain patentably subject matter without regard to otheraspects of the same embodiment. Still further, various aspects ofdifferent embodiments can be combined together. Accordingly, the scopeof the invention should be determined by the following claims and theirlegal equivalents.

What is claimed is:
 1. A method of testing, comprising: performing anatomic force microscopy measurement to obtain a contact resistance valuebetween a pair of thin films when the pair of thin films contact with afirst controlled force applied to at least one of the pair of thinfilms; and performing another atomic force microscopy measurement toobtain a current-dependent stiction force value between the pair of thinfilms after the pair of thin films are brought into contact with asecond controlled force applied to at least one of the pair of thinfilms.
 2. The method according to claim 1 further including the step ofemploying the contact resistance value and the current-dependentstiction force value to determine whether a combination of a firstmaterial used for one of the pair of thin films and a second materialused for another of the pair of thin films will be useable for anapplication of interest.
 3. The method according to claim 2 wherein thestep of employing determines whether the combination will be useable ascontact materials for a microswitch.
 4. The method of claim 3 whereinthe step of determining determines whether the microswitch is usable ina fingerprint sensing array, the method further comprising: determiningan expected sensing force acting on the microswitch during an operationof the microswitch in the fingerprint sensing array, and wherein thestep of performing the atomic force microscopy measurement uses theexpected sensing force as the first controlled force.
 5. The method ofclaim 3 wherein the step of determining whether the microswitch isusable in a fingerprint sensing array, the method further comprising:determining an expected impact force acting on the microswitch during anoperation of the microswitch in the fingerprint sensing array, andwherein the step of performing the another atomic force microscopymeasurement uses the expected impact force as the second controlledforce.
 6. The method of claim 1 wherein the first force is differentfrom the second force.
 7. The method of claim 1 wherein the step ofperforming the atomic force microscopy measurement measures a pluralityof contact resistance values between the pair of thin films at a firstplurality of forces.
 8. The method of claim 7 wherein the step ofperforming the another atomic force microscopy measurement measures aplurality of stiction force values between the pair of thin films at asecond plurality of forces.
 9. The method of claim 7 wherein the step ofperforming the another atomic force microscopy measurement measures afurther plurality of stiction force values between the thin films, eachof the further plurality of stiction force values obtained after adifferent one of a plurality of different electrical currents is passedthrough an interface where the pair of thin films are in contact eachother.
 10. The method of claim 1 wherein the step of performing theanother atomic force microscopy measurement includes passing a currenthigher than 5 μA and lower than 100 μA through an interface where thepair of thin films contact each other using the second controlled force.11. The method of claim 1 wherein the first controlled force is lowerthan 1 mN.
 12. The method of claim 11 wherein the first controlled forceis between 10 μN and 100 μN.
 13. The method of claim 1 wherein thesecond controlled force is lower than 200 mN.
 14. The method of claim 13wherein the second controlled force is between 20 μN and 1 mN.
 15. Themethod of claim 1 wherein the second controlled force is at least anorder of magnitude greater than the first controlled force.
 16. Themethod of claim 1 wherein each film in the pair of thin films has athickness not exceeding about ten microns.
 17. The method of claim 1wherein each of the steps of performing the atomic force microscopymeasurement and performing the another atomic force microscopymeasurement are performed with a first of the pair of thin filmsdisposed on a substrate and a second of the pair of thin films disposedon a rounded piece.
 18. The method of claim 17 wherein the first of thepair of thin films is disposed on a planar substrate.
 19. The methodaccording to claim 17 wherein each of the steps of performing the atomicforce microscopy measurement and performing the another atomic forcemicroscopy measurement include moving the rounded piece to contact thesubstrate and obtain the pair of thin films that are in contact witheach other.
 20. The method of claim 1 wherein a first material of thepair of thin films comprises an alloy.
 21. The method of claim 1,further comprising the step of performing, prior to the steps ofperforming the atomic force microscopy measurement and performing theanother atomic force microscopy measurement, a parasitic resistancemeasurement after the pair of thin films are brought into contact with acalibration force applied to at least one of the pair of thin films, thecalibration force being at least an order of magnitude higher than thefirst controlled force.
 22. The method of claim 1 wherein at least oneof the pair of thin films comprises a multilayer film.